Veritas™ File System Programmer's Reference Guide - Solaris

fcl_iostats structure

VxFS 5.0 and later releases let you gather statistics such as the number of reads and writes occurring on a file. You can enable this through the fiostat command. The gathered stats are maintained in a per-file in-core structure and the File Change Log acts as a persistent backing store for the statistics.

These statistics can be read from the FCL as VX_FCL_FILESTAT records. Each record contains information as defined by the following fcl_iostat structure:

struct fcl_iostats {
    uint64_t nbytesread; /* Number of bytes read from the file*/
    uint64_t nbyteswrite;/* Number of bytes written to the file*/
    uint32_t nreads;     /* Number of reads from the file */
    uint32_t nwrites;    /* Number of writes to the file */
    uint32_t readtime;   /* Total time in seconds for the reads */
    uint32_t writetime;  /* Total time in seconds for the writes */
    struct {
        uint32_t   tv_sec;
        uint32_t   tv_nsec;
} lastreset;/*           Last reset time for the stats */
uint32_tnodeid;          /* Node from which the record was written */
uint32_treset;           /* Stats have been written due to a reset */
};

Each iostat record in the FCL contains I/O statistics accumulated over the time interval from the lastreset time to when the FCL record is written.

For example, computing the aggregate for the total number of reads over a period of time requires traversing a set of FCL files to obtain I/O statistics records. This informations contains a sequence of records of the type VX_FCL_FILESTATS with the same lastreset time followed by another sequence of records with a later lastreset time for a specific file.

The aggregation considers values only from the latest record from records with the same lastreset time and then sums up the number of reads for each such record.